Doesn't need to be a heatpump dryer, just needs to be ventless.
E.g. the 20+ year old common German dryer that runs the clothes air closed-loop, first through the clothes, then to a heat exchanger with drainage geometry (typically feeding into a tray you have to pull out and pour out after each cycle), then across an electric heater, circulation fan, and back to the clothes.
A second fan blows room air across the heat exchanger; the dryer action relies on the clothes being much warmer than room temperature such that the desired relative humidity of/at the clothes doesn't survive the cold from the room temperature.
Yeah, modern heat pump dryers are just nice and have cycle times more in line with North American expectations, but you’re right that alternatives have been around a long time!
E.g. the 20+ year old common German dryer that runs the clothes air closed-loop, first through the clothes, then to a heat exchanger with drainage geometry (typically feeding into a tray you have to pull out and pour out after each cycle), then across an electric heater, circulation fan, and back to the clothes. A second fan blows room air across the heat exchanger; the dryer action relies on the clothes being much warmer than room temperature such that the desired relative humidity of/at the clothes doesn't survive the cold from the room temperature.